Roaming the library, outside of academia

Tag Archives: Thermometers

In this post I will be exploring the rise of scientific instruments and elaborating on some of the ways in which they made empirical science possible. In particular, I will be recounting some of the history of the thermometer.

The 17 century marked the invention of six new scientific instruments: the microscope, the telescope, the thermometer, the barometer, the air-pump and the pendulum clock. These instruments not only gave a means for phenomena to be measured in a way that could be standardised, but also enabled experiments to be carried out in environments which were both controlled and possible to recreate.

One could argue that the invention of these very basic instruments allowed experiments in general to be a viable route towards scientific knowledge. Whewall, when talking about the differentiation between facts and ideas, states the following:

“The impressions of sense, unconnected by some rational and speculative principle, can only end in a practical acquaintance with individual objects; the operations of the rational faculties, on the other hand, if allowed to go on without a constant reference to external things, can lead only to empty abstraction and barren ingenuity”

Before the rise of scientific instruments, we only had knowledge of the world around us by that which we could ascertain by our basic senses. We could easily speculate and theorise about the nature of things, as we see in the writings of ancient philosophers such as Democritus and Lucretius, but we would have no way of being able to solidify the claims we were making. We might have a theory that the apple in front of us was made from tiny indivisible units, but until the invention of powerful microscopes, all we know about the apple is its shape, colour and flavour. As our instruments and methods get better, we are able to refine and reapply our theories.

The story of the thermometer is one which particularly interests me. Very early thermometers and thermoscopes are described from around 1 CE, by writers such as Hero of Alexandria and Philo of Byzantium. These very early thermometers relied entirely on the expansion of air. The thermometer would comprise of a tube which was open at one and and closed by a bulb at the other. This tube was suspended in water, and as the temperature rose the air in the bulb would expand and move the water in the tube. At the time, these instruments were used for pneumatics.

As with much ancient knowledge, thermoscopes were largely forgotten about during the dark ages. It was Gallileo who re-invented the thermometer in around 1592 CE. His thermometer relied on very similar mechanics to the original ones. By around 1612 the first clinical thermometer had been made by Sanctorius. The patient put the top air-filled bulb of the glass tube in their mouth, and the heat then affected the water in the lower end of the tube.

Air thermometers had one big problem – they were affected severely by atmospheric pressure. In 1632 a French doctor, Jean Ray, first suggested creating a liquid thermometer, and between 1641 and 1654 the water used in thermometers was being replaced by alcohol. Some of these thermometers were so large that they needed to be made in a spiral shape to accommodate their length.

At this point, a debate was growing over the best way to standardise thermometers. Previously there had been no standard for marking the temperatures on thermometers and no consensus on how to unitise temperature. Boyle was one of the main instigators for fixing a scale, and suggested that all thermometers should share one fixed point on their scale, this being the freezing point of oil or aniseed. In 1665 Huygens suggested standardising the scale by the proportion between the capacity of the bulb of the thermometer and the bore of the tube, and that either the freezing point or the boiling point of water should be the one fixed point. There were a number of interesting suggestions for what would make a suitable fixed point during the late 1600s, including the melting point of butter.

At some point, the favoured liquid for thermometers became mercury, though it is not known exactly when this happened or exactly who was the first to make a mercury thermometer. In 1714, Farenheit adopted the mercury thermometer and included three fixed points on his scale: the temperature of a mixture of ice, pure water and salt was marked as 0°; the temperature of a mixture of ice and pure water was marked as 32°; the temperature of the human body was marked at 96°. Later, Farenheit added the boiling point of water as a fixed measure, at 212°. Farenheit also added a barometer to accompany his thermometer, as this allowed the user to account for the atmospheric pressure when they were taking a measurement.

The 100 degree scale was invented in 1742 by Celsius. He took his scale as being the melting point of ice as 100° and the boiling point of water as 0°. The term “centigrade” comes from Christin of Lyons.

In this potted history of the thermometer, we can see the shape of the struggles faced by early scientific instruments. Notably, we can see the importance of being able to standardise the scales that these instruments used, and being able to control and account for other factors which might affect our measurements. It took around 150 years of development to have thermometers with a consistent scale measured around a reliable number of fixed points – something that we take entirely for granted today.